Recently, a lithium secondary cell has been commercialized as a secondary cell, and the application thereof has been expanded. However, lithium used in such a lithium secondary cell is not an abundant resource, and there is a concern that lithium resources will be exhausted in the future.
On the other hand, sodium, in the same alkali metal group as lithium, is more abundant than lithium as a resource, and is one digit less expensive than lithium. In addition, sodium has a comparatively high standard potential, and thus it is considered that a sodium secondary cell can be a secondary cell having higher capacitance.
If a sodium secondary cell can be used instead of the current lithium secondary cells, a large secondary cell such as an on-vehicle secondary cell or a secondary cell for distributed power storage can be mass-produced without considering the scarcity of the resource.
In PTL 1 and NPL 1, a sodium-containing composite metal oxide represented by NaFe0.4Ni0.3Mn0.3O2 is disclosed that can be used as an electrode-active material.